Madison Allen (portrayed by Cassandra Sawtell) is Shea and Richard's daughter. She's cute, sweet and, at times, horribly spooky. Lots of kids enjoy mischief; Madison might enjoy it a little too much. She has a tendency to say things that indicate she has a better idea about what's happening on Harper's Island than the adults do. She knows something they don't…something bad. Madison is one of the survivors on Harper's Island.

In Episode 1, Madison scares Chloe and Beth with a severed finger magic trick, revealed to have been tought to her by Cal. She is scolded by her mother, Shea. Shea comments that Madison has no other children to play with on the island, but Richard, her father, says she'll be fine. At the end of the episode, Madison stands over the sleeping Shea in a rather frightening way, and says her new friend told her that people were killed on the island. When Shea asks who told her this, she doesn't respond.